In the (petro)chemical industry as well as during the transport of ecologically unsound substances and at petrol pumps and the like, undesirable vapours or gases may be released. The release of ecologically unsound substances is bound by increasingly stringent environmental regulations. In most cases the admissible concentration of such substances is expressed in p.p.m. (parts per million) or gramme per m.sup.3, whereby the admissible concentration has constantly been reduced over the years.
Furthermore solvents and hydrocarbon represent a relatively high value, which can be retained upon recovery and reuse.
The first known method for recovering solvents and/or hydrocarbons utilize filters, which are mostly provided with active carbon--see for example U.S. Pat. No. 4,583,985, U.S. Pat. No. 3,199,679 and U.S. Pat. No. 2,656,696, whereby certain ecologically unsound substances are adsorbed by the filter material.
Filtering and adsorption units of this type require constant maintenance and attention from operating staff and need to be cleaned at regular intervals. Furthermore such units are liable to wear and must be replaced after some time. Regeneration of the adsorption units usually entails the additional necessity of purifying a water flow. Finally it is by no means certain that the present and future stringent environmental regulations can be met by using filtering units of this type.
Furthermore processes are known wherein use is made of solvents, such as liquid carbon dioxide or chlorofluorocarbons (CFC's), the use of the latter being forbidden at the moment.
Examples of this can be found in U.S. Pat. Nos. 3,664,806 and 3,807,948.
Furthermore a purification system is known for for example benzene, which has a solidification point of 6.degree. C., whereby a gas flow containing benzene is passed through a room cooled to a temperature of -80.degree. C. in order for the benzene to deposit on the walls of a cooling element in the form of a solid substance and/or a liquid, and, after the temperature has been raised and the gas flow has been diverted to a second cooled room--or after a temporary interruption of the gas flow--the benzene can be discharged in liquid form. With this existing unit a pre-cooling unit is commonly used for removing water or water vapour from the gas flow. A process for the removal of condensible compounds from a gas flow is disclosed, for example, in DE-A-4001710.